How did flashing phone charms work ?

They work on a frequency, stick a phone near a speaker and ring it you will here a feedback noise on the speaker just before it rings or text it for similar effect. the noise you hear is the frequency and can sometimes flash even with out being called if it picks up other frequency, these dont work for 3g frequencys on most of these devices or 2100 gsm.

Source :http://wiki.answers.com/Q/How_do_cellphone_charms_detect_that_a_phone_near_them_is_being_called#ixzz1wdZX5Fsu

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